Some vehicles have air suspension systems that are coupled to a reservoir of pressurized air. To raise the vehicle, air from the compressor is fed into one or more air springs of the suspension by way of valves. To lower the vehicle, air is pulled from, or otherwise allowed to escape, the air spring(s).
Some such systems are referred to as open systems, because in them air removed from the air spring is not fed back into the reservoir but rather escapes into the atmosphere. By contrast, a closed suspension system is one in which air from the spring that is being lowered is directed back into the reservoir, typically by running the compressor. That is, compressors in closed systems are operated both when raising and lowering the springs, and therefore typically run more often than compressors in open systems. Nevertheless, closed systems are usually considered more efficient than open systems, because the compressor in the closed system is working against a smaller pressure differential than the one in the open system.
However, running the compressor does generate some noise that can be noticeable, particularly in an electric vehicle where the powertrain noise and vibration are significantly lower than in a vehicle with a traditional combustion engine. Also, the compressor in an electric vehicle is typically powered by the vehicle's internal energy storage (e.g., a battery pack), so that significant compressor use can affect the range of the vehicle between charging sessions.